Recent years have seen an explosive growth in the number of digitally-generated text, image, video, and audio “big data” sources that are increasingly accessible via online archives. The volume of data leads to a need to search for, or to monitor, particular types of content available via open data sources (e.g., social media, content-hosting sites on the Internet) or closed data sources (e.g., standalone archives, seized media). The most commonly used technique to sort through such data is a keyword search, optionally combined with geographical or temporal filters.